1. Field of the Invention
This invention relates generally to the structure and method for making a ceramic lid suitable for use in packaging an erasable programmable read only memory (EPROM) device, and more particularly, to a ceramic lid including an ultraviolet light (UV) transparent lens hermetically sealed thereto, the surfaces of which are substantially free from foreign particulate matter.
2. Related Art
An EPROM circuit is a memory device wherein the memory storage circuits are erasable by irradiation (for example, by UV), and the memory storage circuits can be reprogrammed electrically. EPROM devices must be hermetically sealed in order to be protected from damage and to insure reliability. However, they must also be accessible for irradiation. Thus, EPROM circuits are normally placed within a hermetically sealed protective package that includes a ceramic lid with a UV transparent lens. The UV transparent lens allows the EPROM circuit to be exposed to incoming UV light. EPROM chips within such protective packages are connected to other electronic circuits outside the packages. One such EPROM package is disclosed in U.S. Pat. No. 4,326,214 to R. K. Trueblood. Another type of the EPROM package is shown in U.S. Pat. No. 4,727,221 to Saitou and Bessho, which is concerned with an EPROM device having a shading cover adhered onto a UV window.
Referring to FIGS. 1 and 2, it has been a common practice to hermetically seal a UV transparent lens 10 to a ceramic lid 12 by placing the lens 10 within a preformed aperture 14 of the lid 12 and sandwiching the lid 12 and the lens 10 between paired fixtures 16, 18. The fixtures 16, 18 are each configured to contact the lens 10 and the lid 12 as shown in FIG. 2. An assembly structure 20 results when the fixtures 16, 18, a lens 10, and a lid 12 are fitted together. Upon firing the assembly structure 20, a hermetic seal forms between the lens 10 and the lid 12.
The fixtures 16, 18 are generally made of carbon or graphite because of their basic non-wetting nature with respect to glass, which is often the material of the lens. However, such glass materials are so reactive at high temperatures that they attack both carbon and graphite fixtures, which then tend to disintegrate, flake, or chip. As a result, after firing, there is a considerable amount of foreign particles (principally carbon) deposited on the lens. Also, the surfaces of the lens may be damaged. This particle deposition severely impairs the ability of the lens to pass UV without diffusing it. Therefore, the foreign particles must be removed before the lid 12 and the lens 10 can be used in a semiconductor package. Typically, an alkaline solution is used to clean the lens. However, some of the foreign particles embed in the surface of the lens, and therefore, the alkaline solution cleaning alone does not give satisfactory results; nor can it smooth the surface. On the contrary, the alkaline solution treatment (e.g., boiling a lens in an alkaline solution) roughens the surface of the lens. An additional lens processing step is thus needed. This is normally done by firing the lens again to fire-polish its surface. Although the surface of the lens may be smoothed, the firing does not further eliminate the remaining particles from the lens. This significant level of impurities in an otherwise UV transparent lens is a serious problem where high performance EPROM devices are required.
Accordingly, as the semiconductor industry moves forward to manufacturing more highly integrated EPROM devices, there is an ever-increasing need to minimize or eliminate the noted impurity problems and to provide a ceramic lid having a high grade UV transparent lens suitable for use in a high-density EPROM package.